Large format TV tubes must produce a high peak brightness in order to be acceptable to the user. This results in a large beam current in bright areas of the picture and in a subsequent defocussing of the resulting spot on the display. FIG. 1 illustrates this effect. The horizontal axis 10 scales the vertical line distances and the vertical axis scales the brightness amplitude. A high brightness spot 12 has a significantly increased diameter compared to a low brightness spot 13. The sharpness of a transition between bright and dark areas is therefore impaired due to an overlapping of the bright spot 22 into a dark region as shown in FIG. 2. The dashed line 25 shows the overall response and the hatched area is a region of impairment 24, whereby line 26 marks the position of the edge in the picture.